The gravimeter is widely employed in geological surveying to measure the first derivatives of the earth's gravitational potential function--the gravity field. Because of the difficulty in distinguishing spatial variations of gravity from temporal fluctuations of the accelerations of a moving vehicle, these measurements can be made to sufficient precision for useful exploration only with land-based stationary instruments. This difficulty is in principle avoided by measurement of the second derivatives of the potential--gravity gradients--but only limited success has been met to date in developing a satisfactory gradiometer instrument. Gravity gradiometry is though especially appropriate to the location of geological structures bearing hydrocarbons, to geological mapping, and to locating high density (e.g., sulphides and iron ore) and low density (e.g., potash) mineral deposits.
Although it is not strictly correct to talk about the gradient of gravity, usage of the term has been universally adopted and will be used herein also. More formally, the second derivatives of the gravitational potential are termed gradients of gravity and constitute the gravity gradient tensor with components g.sub.xx, g.sub.xy . . . g.sub.zz, adopting the convention of taking the Z-axis parallel to the local vertical. There are nine such components, only five of which are independent since the tensor is apparently symmetric and the potential is a scalar field obeying Laplace's equation.
The key elements of a gravity gradiometer are a pair of substantially identical spaced masses and the object is to measure differences between the gravitational force on the respective masses. Effectiveness requires measurements of this difference when it approaches only one part in 10.sup.12 of normal gravity. Approaches to measuring gravity gradients have thus far fallen into two broad classes. The first of these entails differential modulation of a signal or parameter by the difference between the gravitationally induced accelerations of the two masses. The second technique involves direct measurement of the net gravitational acceleration of one mass relative to the other.
British patent publication 2022243 by Standard Oil Company discloses a gravity gradiometer in the first class. An element, described in the patent publication as a mass dipole but more properly termed a mass quadrupole, is mounted coaxially on one end of a photoelastic modulator element positioned in the cavity of a ring laser tube to differentially modulate circular polarization modes in response to application of a torque. In a preferred form, two mass quadrupoles are mounted on opposite ends of the modulator element to balance rotational acceleration noise. A closely related development by the same inventor, Lautzenhiser, described in U.S. Pat. No. 4,255,969, employs actual mass dipoles in conjunction with respective photoelastic modulator elements.
Another modulation technique involves rotating a platform which is supporting suitable arrangements of mass pairs. Various instruments of this kind are summarised by Jekeli at 69 EOS (No. 8). One of these, by Metzger, has been further developed and consists of electronically matched pairs of accelerometers on a rotating platform. The platform modulates the sum of opposing acceleration signals with a frequency twice its rotational frequency. These modulation systems call for extremely exacting uniformity in the rotation and require the use of bearing, rotational drive and monitoring technology which is not yet of a standard to render the instruments practicably suitable on an appropriate scale for airborne or moving land-based measurements for geophysical resource exploration, as opposed to geodetic surveying. The alternative of directly measuring gravity gradient components necessitates a very high degree of electronic magnetic thermal and vibration isolation to achieve the measurement accuracy needed. Machines thus far have had poor spatial resolution and a high noise level.
An instrument for measuring the diagonal components g.sub.xx, g.sub.yy and g.sub.zz of the gravitational gradient tensor is described by van Kann et al in the publication IEEE Trans. Magn. MAG-21, 610 (1985) and further elaborated in the NERDDP End-Of-Grant Report (1986) on project No. 738. This instrument consists of a pair of accelerometers mounted with their sensitive axes in line. The difference in displacement of the accelerometers is proportional to the component of the given tensor gradient and is sensed by the modulated inductance of a proximate superconducting coil.
The term "superconducting" is used herein, according to the normal convention, to denote a material which at least is superconducting below a characteristic critical temperature. A suitable such material is niobium, which has a critical temperature of about 9K.
Parent patent application 627820 (48185/90) discloses a gradiometer incorporating a mass quadrupole. The pivotal flexural mounting for the mass quadrupole body may comprise a flexure bearing such as the commercially available Bendix pivot. It has been found, however, that this bearing is less than wholly satisfactory as it is constructed of several different metals secured together and this creates significant problems due to different thermal expansion coefficients and other parameter variations which become critical at the kind of accuracy desired in the present context.